We went to both athletics sessions yesterday.
We arrived at about 8am for the 10am start. It was quiet at Waterloo, and the Jubilee line comfortably busy. We got off at Stratford and it was fine. The moment you get off the train there are enthusiastic games makers anticipating a n exciting day ahead.
You are quickly marshalled into lanes, which makes you think you must be entering the Olympic park straightaway, but it is a good 20 minute walk. Security is so massive that we never stopped to queue at all - we just kept on walking. They said to empty all liquids before the first metal detector. As we didn't have any, that didn't apply to us. Your bags go into a plastic tray, and you are asked to empty pockets. You do not have to open or take anything out of your bag, or remove belts and shoes, as you would at the airport.
We had breakfast in a real restaurant, located across from the Park Live big screen. Surprisingly, none of the kiosks we saw had traditional breakfast foods, but we were able to sit down and have a full English in the restaurant.
There wasn't a queue for the megastore when we went in. The merchandise looked about the same price as the Olympics store in John Lewis.
There were queues for some water points, and others were empty. Don't feel you have to stand in a long queue. In the stadium area, the taps are all walk-up.
We picked up sandwiches in Waterloo, but more because our athletics session went on till almost 2pm. I didn't find the prices outrageous when compared with theme parks, for example. We figured we were there and not going to spoil the day by penny-pinching, so did pay rather a lot for a couple of bottles of wine and some beers (if you want a beer get a draught pint for the same price as a bottle).
Leaving at the end of the day was obviously crowded, but so well marshalled that it could go wrong. Mobility and prams had their own lanes. They were packing the Jubilee trains, but if you need to get on with a buggy, I would suggest just waiting for a few minutes to get on first on a new train, and head along the platform away from the crowds.
If you are travelling from outside London, buy your train ticket online at a special Games fare. You get a very good discount and can combine it with your F&F railcards and then spend the savings on overpriced wine.